a. Field of the Invention
The present invention relates to discrete thin film strain gauge devices, to devices utilizing strain sensitive surfaces, and to methods of manufacturing such devices.
b. Definition(s)
As used here with reference to the present invention, "transparent" includes non-visual translucent. Also, "transparent" includes "translucent" and vice versa.
c. Current State of the Relevant Technology
To my knowledge, conventional strain measurement technology involves two relevant types of devices.
First, conventional strain gages use a straight or curved conductor element (for example, a serpentine conductor) whose electrical resistance varies with strain or pressure. The opaque sensor element comprises metal wire, semiconductor material or a foil of metal such as copper.
The second application is to strain sensitive surfaces, which may comprise an xy grid of conductors separated by a pressure sensitive, variable resistance film or membrane. The opaque membrane provides variable electrical output (resistance and current) in the circuit paths defined through the membrane at the crosspoints of the opaque x and y conductors. Preferably, the membrane has linearly variable, repeatable resistance as a function of pressure.
U.S. Pat. No. 4,734,034, issued Mar. 29, 1988 to Maness et al, discloses the use of strain surfaces in an occlusal diagnostic device for displaying the points of contact between a patient's upper and lower teeth. The sensor input device comprises top and bottom layers, each containing an array of parallel conductors. The x and y conductors are formed by opaque metal lines or conductive inks and are separated by a conductive/resistive layer. When the flat sensor input device containing the layered composite is positioned in a patient's mouth between the teeth, biting the device compresses the separation layer and this decreases the resistance between an associated crosspoint of the xy conductor arrays. The resistance of the separation layer effects a switching action such that when the resistance is decreased below a threshold value, current flows through the upper and lower conductors at the associated crosspoint causing the associated crosspoint location to be displayed on the system monitor as one of the contact points within an outline of the patient's teeth.
Another example of the application of the xy strain surface technology is in so-called digitizing pads which are used in CAD (computer assisted design) systems. The digitizing pads are used for inputing information such as lines or circles to a computer, which manipulates or otherwise operates on the manually-generated input. Here, as in the above systems, the xy conductors and the supporting resistive layer(s) are opaque.
It is highly desirable to mount such xy-conductor digitizing pads directly on the CRT monitor. Obviously, such an approach requires a large (for example, 14 in. square) strain sensitive transparent device. In addition, it is highly desirable to mount strain or pressure gages on glass panels and windows, for example to provide security for homes, offices, display cases, etc. However, such uses are not practical with the existing strain measurement technology, which uses opaque materials.
To my knowledge, the prior art strain sensitive surface technology is limited to the peripheral conductor approach disclosed in Ng et al U.S. Pat. No. 4,476,463 issued Oct. 9, 1984. The Ng '463 patent uses a peripheral array of electrodes along the different edges of a faceplate (such as a CRT or monitor faceplate) to measure the impedances of the faceplate surface. The peripheral conductors do not interfere with the central viewing area of the faceplate.